The present invention relates to imaging of the interior of a body using computed tomography (CT). Onboard imaging (OBI) system with kilovoltage X-ray cone beam computed tomography (CBCT) capability allows two-dimensional (2D) and three-dimensional (3D) image guidance for treatment positioning in a body. An X-ray cone beam source is mounted on a gantry that directs the source in multiple directions through a body. The set of acquired projection images are used to infer the 3D distribution of X-ray attenuation in the body using the well known and established principles of computed tomography.
During some procedures it is important to have near real time position information for structures within the body, but full dose 3D imaging is time consuming and increases the exposure of the body to potentially harmful X-ray radiation. Therefore low dose 2D images often are taken for positioning during treatment. Unfortunately, such images include the effects of every structure within the body and contrast for a particular feature of interest is typically lower than in a slice of the full 3D CT imaging.